Koshi Castle (古志城) is a stage exclusive to the Warriors Orochi series. It serves as the final stage for all story scenarios in Warriors Orochi. Though Koshi is a district in Japan, the castle is fictional as there are no historical records of it.

The castle is implied to be Orochi's base of operations as the armies fight here for a decisive battle against the snake demon. Koshi Castle is a hidden fortress situated in the bleak wasteland of a semi-active volcano caldera. Surrounded by four cannon bases and garrisons at each corner of the map, the castle is well defended. The gates are surrounded by two drawbridges over a moat of molten lava, which block intruders from entering. When the defenses are ruined, Lu Bu will often serve as the final guardian before reaching the inner keep.

In Warriors Orochi 2, the place is not visited as frequently. In Shu's story mode, Kiyomori uses this location to conduct a ceremony to resurrect his lord. The expansion pack also features the castle as Orochi X's Dream Mode stage where he and his cohorts clash against the coalition in an all-out showdown.

A new Koshi Castle appears in the final chapter of the third game, but this area bears little resemblance to its predecessor. Note that there are two variations of that stage: one in which three devices must be destroyed in order to open the castle's inner gate while another adds two more for the outer gate. Inside, the Hydra will attempt to attack any intruder reaching the throne room. The original version of Koshi Castle is used for three stages only available as downloadable content.